Hello this is my first post to the forum. I am using a software called Thaiphoon Burner to change some XMP profile on my RAM but I cannot find the "SPD Write" function in my bios settings. I've had an ASUS board that allows me to enable this function and I've change my XMP on that board.
When I tried it on my Gigabyte board it tries to flash XMP but reverts to the original. I am wondering if anyone could help me to mod my Gigabyte bios to enable the SPD Write function.

Was the Asus DDR4 and Intel board? Many DDR4 can't be written due to chip on memory stick is write protected, so even with that setting changed it wont be possible. Also, I've only seen this on Intel board, not AMD "SPD Write Protect - Default True"
I checked your BIOS F51e AND F1, this setting is not there, you will have to use another board and edit it's BIOS to change this setting to default first too. As mentioned, I've only seen this on Intel board, but I don't look for it often, so maybe miss it sometimes on some AMD.

Yes the Asus board was an Intel motherboard. I also had a Asus Prime x370-PRO a couple years ago that allowed me to modify XMP on my GSkill Sniper RAM without any changes to the BIOS. I wasn't sure if there was a setting to enable SPD Write on my Gigabyte B450 board since it's the only one that restricts me from writing new XMP profile using Thaiphoon Burner. It might be a different setting for AMD but I have no clue what it might be.

Yes, as mentioned, this is a common (hidden) setting for Intel, not sure if I've ever seen in AMD BIOS. You'll have to use Intel board, or whatever AMD board you know that allows it without BIOS mod, I don't see any hidden setting similar to this in your B450 BIOS.